The present invention relates to a copier which judges the kind of document image, such as a color image, a white and black image, a character image/a photographic image and density, according to the image data outputted from a scanner, and its which processes the image data according to the results of the judgement.
In a conventional copier, the following image processing method has been proposed: a document image is pre-scanned before transfer; sampling is conducted on the image data outputted from the scanner; the kind of document image, such as a color image/a white and black image, a character image/a photographic image and density, is judged according to the image data obtained by the sampling; and its image processing is conducted in accordance with the results of the judgement.
The density histogram illustrated in FIG. 5 is made from the sampled image data.
In the density histogram described above, two peaks usually appear at the position of the average density in the background and at the position of the average density of the letter portion. These two peaks are clear in the case of a document of character images. However, they are not clear in the case of a document of photographic images. Consequently, whether the image is a character one or a photographic one can be judged according to the shape of the density histogram.
The position of the peak in the density histogram corresponds to the density of the document, so that the density of the document can be judged according to the position of the peak in the density histogram.
In Japanese Patent Publication Open to Public Inspection No. 194680/1989, a method to set up a threshold value is disclosed which is characterized in that: the kind of an image is judged by making a density histogram in the manner described above; and the image data is quantized according to the judgment results.
For another example, the sampled data of red, green and blue is converted into values of the L*,a*,b* color specification system; when the values are in the chromatic region set up in the L*,a*,b* color specification system as illustrated in FIG. 6, color codes are generated; when the number of the color codes is not less than the threshold value, the document image is judged to be a color image; and when the number of the color codes is smaller than the threshold value, the document image is judged to be a white and black image. The color codes are generated, for example, by a ROM table illustrated in FIG. 7.
In the manner explained above, the development time of white and black images can be reduced by judging whether the document image is a color one or a white and black one. This method is disclosed in Japanese Patent Publication Open to Public Inspection No. 163970/1984.
As explained above, the sampling region of the image data by which the kind of an image is judged is conventionally fixed to all the region of a platen (for example, the region of A3 paper size) or fixed to a portion thereof (for example, the region of B6 paper size).
The document region to be transferred onto a transfer paper differs according to the size of the transfer paper and the magnification of transfer. For example, even when the size of a document is the same as that of a transfer paper, only a portion of the document is transferred on the transfer paper when magnified transfer is performed.
In other words, in the case of the conventional method in which the sampling region of the image data by which to judge the kind of an image is fixed, there is a fear of misjudgment because in some cases the image data of a document region which is not transferred onto a transfer paper is used for judgment.
It is a primary object of the present invention to provide a copier in which the image to be copied ca be properly judged.